The invention relates to a process and a device for increasing the intrinsic viscosity of a polyester material by solid-state polymerization, wherein the polyester material is heat-treated in a heat treatment container.
When producing high-molecular polyesters such as PET and PEN, for example, a behavior of polyester which is unique among synthetic materials is utilized, according to which behavior polycondensation of the polyester molecules occurs and hence the viscosity of the polyester is increased if the polyester remains under high temperatures and vacuum or under inert gas in order to prevent the oxidative degradation. This preparation of high-molecular polyesters from a low-molecular polyester starting material usually occurs via melt polymerization or solid-state polymerization or a combination of both processes.
In case of melt polymerization, a polyester melt is processed at temperatures of approx. 270° C. to 300° C. for about 30 minutes to 5 hours under a strong vacuum of approx. 1 mbar. This involves the drawback that, due to the high processing temperatures, the initially described oxidative degradation process of the polyester will take place which leads to yellow coloring and counteracts the polycondensation of the polyester. The intrinsic viscosity values achievable by melt polymerization are approximately in the range of 0.6 IV (=Intrinsic Viscosity).
In case of solid-state polymerization, the polyester melt is usually extruded through several dies, and subsequently the synthetic strands thereby formed are cooled in a water bath. After having solidified, the synthetic strands are granulated, i.e. cut into pellets. Due to the rapid cooling, the polyester is provided in the amorphous state. This is important since polyester materials which originally were transparent remain translucent in the amorphous state, whereas, if cooling is slow, the polyester assumes a crystalline state in which a material which originally was transparent changes its color to white. For further processing, the polyester granulate must be reheated, whereby the granulated bodies become agglutinated in the range of the crystallization temperature (80-120° C.). Therefore, the granulate is first supplied to a so-called crystallizer in which it is brought to a temperature above the crystallization temperature under vigorous stirring in order to regain the flowability of the granulated bodies for further treatment, which is of great importance for the conveyance and drying in a container without agitator. Moreover, the granulate, in its crystalline form, absorbs less moisture, thereby permitting shorter residence times during drying. The granulate is then fed into a solid-state polymerization container, also referred to as an SSP (solid-state polymerization) reactor or heat treatment container, wherein it is heated to approx. 220 to 250° C. and subsequently is left under those conditions for about 1-40 hours until the desired intrinsic viscosity has been reached.
The heating of the polyester granulate in the SSP reactor is carried out according to the prior art either by means of an inert gas stream (f.i. nitrogen) as a heat-transfer medium, which stream—which is heated outside of the reactor—passes through the reactor and the granulate located therein, thereby transferring its heat to the granulate, and subsequently is sucked off, or by means of heating elements in an evacuated reactor.
Heating by an inert gas stream involves the disadvantage that the employed technical gases (f.i. nitrogen) are expensive and therefore must be conducted in a closed circuit, also for reasons of environmental protection. The closed circuit also requires that costly cleaning devices for the inert gas stream must be provided in order to filter out toxic substances and impurities taken up from the granulate. The implementation of such a reactor therefore only pays off with huge plants having a throughput in the range of 20 tons of polyester granulate per day and more.
Heating the granulate in a container under vacuum involves disadvantages in that the vacuum is an excellent heat insulator and, due to this property, counteracts the heating of the granulate. Therefore, it is either necessary to provide extremely long residence times of the granulate in the container or, in case of heating via heating elements attached to the exterior of the container, to provide one or several agitators for mixing the granulate in the interior of the container, or to provide technically complex movable heating elements in the interior of the container, which serve simultaneously as mixing elements. All those constructional measures, however, give rise to technical problems such as the formation of dead spaces in which the granulate gets stuck, turning round of the granulate stream, non-uniform heating, high energy consumption, etc. and are undesirable also for cost reasons. For the above-mentioned reasons, a continuous charging of the heating container with granulate is rendered extremely difficult.